The present invention relates to a radio channel control method for connecting a plurality of stationary apparatuses and a plurality of portable mobile telephones through radio channels and, more particularly, to a radio channel control method in a mobile communication apparatus such as a cordless telephone used in a relatively small area.
FIG. 1 is a block diagram showing an arrangement of a cordless telephone set. In FIG. 1, reference numeral 1 denotes a main apparatus connected to a telephone line and having a line control function; 10.sub.l to 10.sub.n, a plurality of stationary apparatuses each having a radio receiver/transmitter; and 20.sub.l to 20.sub.n, portable telephones. The main apparatus 1 and the stationary apparatuses 10.sub.l to 10.sub.n are couple L.sub.l to L.sub.n. The portable telephones 20.sub.l to 20.sub.n and the stationary apparatuses 10.sub.l to 10.sub.n are present in a single area A and use a single frequency as a control radio channel. For this reason, the main apparatus 1 is set in a main state in which only a stationary apparatus designated by a control signal is controlled through a radio channel. Therefore, the remaining stationary apparatuses are set in a sub state in which they wait for designation by the control signal.
In the conventional radio channel control method, however, when a control signal cannot be received even once due to an error in a wire line between the main apparatus 1 and the stationary apparatus, all the stationary apparatuses are set in the sub state, and the radio channel cannot be connected. This drawback will be exemplified below.
FIG. 2 is a chart showing a conventional radio channel control method. For the sake of simplicity, an arrangement comprises the main apparatus, first and second stationary apparatuses, and first and second portable telephones. Assume that the first stationary apparatus is designated in the main state by the main apparatus.
In FIG. 2, the first portable telephone transmits a call signal S.sub.21 to the first stationary apparatus in response to a call operation by a user. Upon reception of the call signal S.sub.21, the first stationary apparatus designated in the main state transmits a call signal S.sub.22 to the main apparatus, and transmits a call response signal S.sub.23 including communication channel information to the first portable telephone. Thereafter, the first stationary apparatus is switched to the sub state by itself. Subsequently, upon reception of the call response signal S.sub.23, the first portable telephone is switched to a communication channel, and transmits a channel switching signal S.sub.24 to the first stationary apparatus. When the first stationary apparatus receives the channel switching signal s.sub.24, it transmits an AF (Audio Frequency)-ON signal S.sub.25 to the first portable telephone, and transmits a connection end signal S.sub.26 to the main apparatus, thus enabling communication with the first portable telephone. Meanwhile, when the main apparatus receives the call signal S.sub.22, it transmits a state designation signal S.sub.27 to the second stationary apparatus to switch the second stationary apparatus to the main state. Upon reception of the connection end signal S.sub.26 from the first stationary apparatus, the main apparatus connects the first portable telephone and the telephone line.
However, when a given error occurs in the wire line and the second stationary apparatus cannot receive the state designation signal from the main apparatus, the second stationary apparatus is kept in the sub state. Therefore, in this state, even if a user of the second portable telephone transmits a call signal S.sub.28, the radio channel cannot be connected since the second stationary apparatus is kept in the sub state.
As described above, in the conventional method, since the main apparatus does not confirm the states of the stationary apparatuses, if a given error occurs in the wire line and the state designation signal cannot be received even once, the radio channel cannot be connected.